In the past, in the process of manufacturing semiconductor devices, a two-step soldering process is carried out as a soldering method using a Sn—Pb based solder alloy, which is a typical solder joint material. Specifically, in the conventional soldering process, a Pb-5Sn solder (melting point: 310 to 314° C.) is primarily used in a conventional semiconductor device to conduct the soldering and, in the following reflow soldering for jointing the semiconductor per se to a substrate, a temperature-layered joint is used including a process of conducting the joint with a Sn-37Pb eutectic solder having a low melting point (melting point: 183° C.) so as not to melt the soldered portion already jointed in the semiconductor device.
On the one hand, in view of environmental problems, it is required in Restriction of the use of Certain Hazardous Substances in electrical and electronic equipment (RoHS) and the like to take steps for making solders Pb-free, and a Sn—Ag—Cu based eutectic alloy has been put into practical use as a reflow solder. In regard to high-temperature solders, on the other hand, an Au-20Sn solder (melting point: 280° C.) is known, but is hardly used since the Au-20Sn solder is inferior in cost and mechanical properties to Pb—Sn based solders. Since other component systems have not yet been put into practical use either, high-temperature solders containing Pb still remain exceptional in RoHS.
In regard to development of high-temperature solder alloys, for example, Japanese Patent Laid-Open Publication No. 2003-260587 describes a Sn—Cu based solder. This technique proposes a method which includes using a material prepared by mixing Sn powders and Cu powders together as a soldering paste and, upon high-temperature soldering, melting the Sn powders to contribute to the soldering as well as to react with the Cu powders to form a Sn—Cu intermetallic compound phase having a high melting point. This compound phase functions, upon reflowing, to retain the strength of the soldered portion without melting.
The above conventional techniques, however, have problems that a structure of an intermetallic compound phase formed in phase boundaries between the Sn powders and the Cu powders is formed on the surface of the Cu powder due to size of approximately 10 μm of the practically used powder to result in coarse structure and variation in strength and that a large amount of the Cu powder has to be used for increasing its strength to result in deterioration in the soldering properties provided by the Sn powder. Further, since formation of the intermetallic compounds relies upon a diffusion reaction between Sn in a molten liquid state and Cu in a solid state, it takes time to conduct this reaction due to a slow formation speed of a Cu6Sn5 intermetallic compound phase contributing to retention of strength, causing a disadvantage in the producing process.